package org.apache.commons.graph.weight

/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements.  See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership.  The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License.  You may obtain a copy of the License at
*
*   http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied.  See the License for the
* specific language governing permissions and limitations
* under the License.
*/

/**
 * A [Monoid] is a [Semigroup] with an identity value.
 *
 * @param <E> the type of the elements in the [Monoid]
</E> */
interface Monoid<E> {
    /**
     * Returns the result of the associative binary operation defined by this
     * [Semigroup] between two elements of appropriate type.
     *
     * @param e1 the first element
     * @param e2 the second element
     * @return the result of the associative binary operation
     */
    fun append(e1: E, e2: E): E

    /**
     * Returns the identity value.
     *
     * @return the identity value
     */
    fun identity(): E

    /**
     * Returns the inverse of the input element.
     * @param element the input element
     * @return the inverse of the input element
     */
    fun inverse(element: E): E

    companion object
}
